In a natural circulation vertical heat recovery steam generator (HRSG) the evaporator and circulation loops do not have pump circulation. The evaporator and circulation loops rather than having a circulation pump are dependent on natural circulation during operation. Natural circulation is driven by a density difference between the fluid in system downcomer(s) and the fluid in system riser(s) or in system riser(s) and evaporator tubes. During steady state shutdown conditions, i.e., cold, warm and hot shutdown, there is almost no natural circulation, and fluid in the circulation loop is stagnant. After starting a gas turbine associated with the system, the fluid in the evaporator tubes is heated by a hot gas from the gas turbine and eventually starts to boil. In theory, when the fluid in the evaporator tubes begins to boil, natural circulation should be set flowing within the circulation loop in the “correct” direction, i.e., from a steam drum, through the downcomer(s), the evaporation tubes, the riser(s) and back to the steam drum.
The initiation of natural circulation in the correct direction from stagnant fluid conditions is very important for proper operation of the HRSG. In a horizontal HRSG, the evaporator tubes are typically arranged in a vertical orientation. With evaporator tubes in a vertical orientation, natural circulation within the circulation loop is established in the correct direction without difficulty. However, in the subject vertical HRSG, the evaporator tubes are arranged in a horizontal or substantially horizontal orientation. With evaporator tubes in a horizontal or substantially horizontal orientation, the onset of natural circulation within the circulation loop may not immediately establish in the correct direction. Steam produced in the horizontal oriented evaporator tubes may, at least at the onset, flow in an undesirable “reverse” direction, i.e., from a steam drum, through the riser(s), the evaporation tubes, the downcomer(s) and back to the steam drum. Such reverse flow is undesirable as the flow could lead to severe drum level transients during startups, or, in some cases, lead to cessation of fluid circulation in the circulation loop for extended periods of time. This risk is especially possible during a cold startup. Accordingly, a system to initiate natural circulation in the correct direction in horizontally oriented evaporator tubes of a vertical HRSG at startup is needed. Likewise, a method of initiating natural circulation in the correct direction in horizontally oriented evaporator tubes of a vertical HRST at startup is needed.